1. Field of the Invention
Embodiments of the present invention relate to control methods and control apparatuses of internal-combustion engines. Internal-combustion engines typically have a turbo supercharger. A turbo supercharger has variable nozzles capable of adjusting the flow velocity of gas exhausted to a turbine impeller.
2. Description of the Related Art
In recent years, internal-combustion engines have been widespread. The engines have a turbo supercharger including variable nozzles. Adjusting an opening degree of the variable nozzles may obtain an appropriate supercharging pressure even if the number of revolutions of the internal-combustion engine is low. Thereby, output power of the internal-combustion engine may be increased.
When the internal-combustion engine is operated in very high load and high revolution states, a big pressure difference occurs before and after a turbine impeller. A stress occurs before and after the turbine impeller due to the pressure difference as well as exhaust pulsation. Exhaust pulsation may occur due to each explosion stroke of the internal-combustion engine. If the turbine impeller is continuously exposed to a stress which is equal to or greater than a predetermined stress for a long time, the life thereof may be shortened. A method for preventing the above problem is known. In the method, the engine is controlled so that a before-after pressure ratio of the turbine impeller is equal to or less than a predetermined pressure ratio. The before-after pressure ratio is pressure before turbine/pressure after turbine.
A control apparatus of an internal-combustion engine disclosed in Japanese Laid-Open Patent Publication No. 2010-31829 includes a target upstream side exhaust pressure calculator and a vane opening degree controller in order to improve feedback control of a supercharging pressure. The target upstream side exhaust pressure calculator calculates a target upstream side exhaust pressure such that actual supercharging pressure matches a target supercharging pressure. The vane opening degree controller calculates a target before-after exhaust pressure ratio such that actual upstream side exhaust pressure matches the target upstream side exhaust pressure. The vane opening degree controller determines a vane opening degree (variable nozzle opening degree) corresponding to the target before-after exhaust pressure ratio on the basis of a predetermined turbine model.
The internal-combustion engine includes an Exhaust Gas Recirculation (EGR) in order to reduce NOx in exhaust gas. Pressure sensors are respectively provided on the upstream side and the downstream side of the turbine. The target before-after exhaust pressure ratio is obtained using the actual upstream side exhaust pressure which is a pressure on the upstream side of the turbine. When the EGR is turned from an OFF state to an ON state, an opening degree of the variable nozzles instantaneously varies. However, the EGR amount and the actual upstream side exhaust pressure vary slightly shortly after. Due to this response delay, there is a probability that an exhaust pressure ratio before and after the turbine may exceed an allowable value only for a short time. For this reason, the turbine impeller cannot be sufficiently protected in some situations.
A before-after exhaust pressure ratio (actual upstream side exhaust pressure/actual downstream side exhaust pressure) is practically obtained, and control according to a difference with the target before-after exhaust pressure ratio is performed. For this reason, the actual downstream side exhaust pressure is necessary for the control. Thus, a pressure sensor is necessary on the turbine downstream side. A temperature sensor is also necessary on the turbine upstream side.
Therefore, there is need in the art for a control method and a control apparatus of an internal-combustion engine, capable of appropriately controlling a before-after pressure ratio of a turbine impeller to a desired pressure ratio with good responsiveness. There is also the need in reducing the number of detection means (sensors).